Opaque result types

[Moximillian (Mox)]

@zwaldowski

Given the Collection confusion — that it's purportedly not a good example for how we'd want to use this first version of opaque types, and that ABI stability saddles the stdlib with its LazyFilterFlattenCrunchyRawUnbonedCollection types — what is the intended, platonic ideal use case? There are lots of examples from forum users about how they'd like to use them, which is good and valuable, but not the same.

EDIT:
This comment from @nuclearace is enough to convince me that there is a real need for Opaque result types beyond Collection in Foundation. So it makes sense to have a proposal .